The "where" statement in MySQL will interrupt the full connection

Question

Consider the following table:

create table `t1` (
  `date` date,
  `value` int
);

create table `t2` (
  `date` date,
  `value` int
);

insert into `t1` (`date`, `value`)
values ("2022-01-01", 1),
       ("2022-03-01", 3),
       ("2022-04-01", 4);
       
insert into `t2` (`date`, `value`)
values ("2022-01-01", 1),
       ("2022-02-01", 2),
       ("2022-04-01", 4);

t1 table is missing 2022-02-01 date, t2 table is missing 2022-03-01. I want to join these two tables to produce the following result:

| t1.date    | t1.value | t2.date    | t2.value |
|            |          |            |          |
| 2022-01-01 | 1        | 2022-01-01 | 1        |
| null       | null     | 2022-02-01 | 2        |
| 2022-03-01 | 3        | null       | null     |
| 2022-04-01 | 4        | 2022-04-01 | 4        |

The solution is to use full connection:

select *
from `t1`
left join `t2` on `t2`.`date` = `t1`.`date`

union

select *
from `t1`
right join `t2` on `t2`.`date` = `t1`.`date`;

This can get the results I want. But using the where statement breaks everything:

select *
from `t1`
left join `t2` on `t2`.`date` = `t1`.`date`
where `t1`.`date` > "2022-01-01"

union

select *
from `t1`
right join `t2` on `t2`.`date` = `t1`.`date`
where `t1`.`date` > "2022-01-01";

I expected to get this result:

| t1.date    | t1.value | t2.date    | t2.value |
|            |          |            |          |
| null       | null     | 2022-02-01 | 2        |
| 2022-03-01 | 3        | null       | null     |
| 2022-04-01 | 4        | 2022-04-01 | 4        |

But I got this result:

| t1.date    | t1.value | t2.date    | t2.value |
|            |          |            |          |
| 2022-03-01 | 3        | null       | null     |
| 2022-04-01 | 4        | 2022-04-01 | 4        |

I know what's wrong but can't find a solution. The problem is that t1.date > "whatever" filters out all empty rows in the t1 table. I've tried this method but it doesn't work:

where `t1`.`date` > "2022-01-01" or `t1`.`date` = null

Answer 1

You should use

where `t1`.`date` > "2022-01-01" or `t1`.`date` is null

"NULL = NULL" evaluates to false because NULL has no value. Therefore it cannot be the same as any other value (even another NULL). The correct way is to use is null

Answer 2

It seems you should use t2.date > "2022-01-01" in the right join query.

select *
from `t1`
left join `t2` on `t2`.`date` = `t1`.`date`
where `t1`.`date` > "2022-01-01"

union

select *
from `t1`
right join `t2` on `t2`.`date` = `t1`.`date`
where  `t2`.`date` > "2022-01-01";

View the demo at https://dbfiddle.uk/reo8UanD.